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A. KURC-LISIECKA, A. LISIECKI: LASER WELDING OF THE NEW GRADE OF ADVANCED HIGH-STRENGTH STEEL ...199–204

LASER WELDING OF THE NEW GRADE OF ADVANCEDHIGH-STRENGTH STEEL DOMEX 960

LASERSKO VARJENJE DOMEX 960 NOVEGA NAPREDNEGAJEKLA Z VISOKO TRDNOSTJO

Agnieszka Kurc-Lisiecka1, Aleksander Lisiecki2

1University of D¹browa Górnicza, Rail Transport Department, Cieplaka 1C, 41-300 D¹browa Górnicza, Poland2Silesian University of Technology, Faculty of Mechanical Engineering, Welding Department, Konarskiego 18A, 44-100 Gliwice, Poland

[email protected], [email protected]

Prejem rokopisa – received: 2015-07-01; sprejem za objavo – accepted for publication: 2016-02-09

doi:10.17222/mit.2015.158

The article presents the results of investigations on autogenous laser welding of the new-generation advanced high-strengthsteels (AHSS) Domex 960, classified as thermomechanically rolled steel by the manufacturer. There is little information aboutwelding this steel grade, relating only to arc-welding methods. Therefore, a modern disk laser was used for the butt-joint weld-ing of 5.0 mm steel sheets. The results of a microstructure study, tensile tests, technological bending tests, impact-toughnesstests and also microhardness measurements showed that a low heat input during the laser welding of the new Domex 960 gradesteel is advantageous. A low heat input and thus a high cooling rate of the weld metal and the heat-affected zone (HAZ) led tothe formation of a favourable fine-grained microstructure, providing also high mechanical properties of butt joints, comparableto the properties of the base metal. Despite the high cooling rates, there was no significant increase in the microhardnessmeasured across the butt joints. Moreover, a slight decrease in the microhardness was observed in the HAZ.Keywords: laserlaser welding, disk laser, thermomechanically rolled steel, advanced high-strength steel, fine-grained steel, pro-perties of welded joints

^lanek predstavlja rezultate preiskav avtogenega laserskega varjenja nove generacije naprednih visokotrdnostnih jekel (AHSS),jekla Domex 960, ki so ga proizvajalci opredeljujejo kot termomehansko valjano jeklo. Malo je informacij o varjenju te vrstejekla, nekaj le za metode oblo~nega varjenja. Za sole`no varjenje 5 mm debelih plo~evin je bil uporabljen modern diskasti laser.Rezultati {tudija mikrostrukture, nateznih preizkusov, tehnolo{kih upogibnih preizkusov, preizkusov udarne ìlavosti in tudimeritev mikrotrdote, so pokazali, da je ugoden majhen vnos toplote med laserskim varjenjem nove vrste jekla Domex 960.Majhen vnos toplote in zato velika hitrost ohlajanja kovine v zvaru in v toplotno vplivani coni (HAZ), povzro~i nastanek èljenedrobno zrnate mikrostrukture in tudi zagotavlja visoke mehanske lastnosti sole`nega zvara, v primerjavi z lastnostmi osnovnekovine. Kljub velikim hitrostim ohlajanja, ni bilo ob~utnega pove~anja mikrotrdote, izmerjene preko sole`nega zvara. Poleg tegaje bilo v HAZ opaèno rahlo zmanj{anje mikrotrdote.Klju~ne besede: lasersko varjenje, diskasti laser, termomehansko valjano jeklo, napredna visokotrdnostna jekla, drobno zrnatojeklo, lastnosti zvarjenih spojev

1 INTRODUCTION

The constant trend to increase production efficiency,reduce energy consumption and material consumption,improve the durability and reliability has lead to anincreasing use of advanced high-strength materials.1–8

Depending on the application, working conditions andthe type of load and wear, different metallic, bimetallicor composite materials are used.9–12

In the case of the manufacturing of building struc-tures such as bridges, towers, industrial buildings andvehicles, particularly heavy vehicles such as trucks,trailers, semi-trailers, rail vehicles such wagons andtramcars, and also other utility vehicles and machinessuch as cranes, loaders, excavators, etc., the primarygroup of materials is structural steel.13–18 The reason forthis is the ease of forming and joining steel pieces,usually with welding technologies. Moreover, the use ofhigh-strength steel (HSS) has been growing in theindustry for many years. The dynamic development ofhigh-strength steels has led to the introduction of modern

advanced and ultra-high-strength steels (AHSS andUHSS), characterized by an extremely high yield pointand tensile strength over 1000 MPa.1–8 Steels of this typeachieve high mechanical properties thanks to a veryfine-grained structure and/or a sophisticated phase com-position like in the case of dual-phase (DP), complex-phase (CP) and transformation-induced plasticity (TRIP)steels.2,6,7

A significant group of the modern structural steelscurrently used in the industry includes thermomechani-cally rolled fine-grained microalloyed steel classified bythe EN 10149-2 standard and also the quenched andtempered low-alloy steel classified by the EN 10025-6standard. The EN 10149-2 standard covers the thermo-mechanically rolled fine-grained microalloyed steel witha yield point of up to 700 MPa (e.g., S700MC). Anexample of the commercial name of this type of steel isDomex 700, manufactured by a Swedish company. Onthe contrary, the EN 10025-6 standard covers thelow-alloy quenched and tempered steel with a yield point

Materiali in tehnologije / Materials and technology 51 (2017) 2, 199–204 199

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of up to 960 MPa (e.g., S960QL). An example of thecommercial name in this case is Weldox 960.

Steels of the above groups differ in the chemicalcomposition and manufacturing procedure, even if theyhave similar mechanical properties, as shown in Table 1.Differences in the chemical composition affect the car-bon equivalent Ce, thus the hardenability and the asso-ciated susceptibility to cold cracking (hydrogen crack-ing).15,20–22 Beside the chemical composition, structuralconstituents, especially the strengthening precipitationslike carbides, borides or nitrides, also influence therequirements and specification of the welding procedure.

Table 1: Chemical composition of S700MC steel according to EN10149-2

Tabela 1: Kemijska sestava jekla S700MC, skladno z EN 10149-2

C Si Mn Al Nb V Ti Mo B0.12 0.6 2.1 0.015 0.09 0.2 0.22 0.5 0.005

CEV max = 0.38, CET max = 0.25

As a result of an intensive research and a dynamicdevelopment of modern steel grades, the world’s largeststeel manufacturers now offer steels with properties thatgo beyond the standards. Examples are steel gradesWeldox 1100 or Weldox 1300 classified as quenched andtempered steel, suitable for welding. Additionally, newsteel grades, such as Domex 960, are available in themarket. The Domex 960 steel grade is classified by themanufacturer as a thermomechanically rolled steel eventhough the properties of this steel are at the level ofquenched and tempered Weldox 960.

Thus, with the introduction of new grades of steel,the classification becomes more difficult.

The reason for this is a very sophisticated chemicalcomposition, the combination of individual elements andalso the sophisticated manufacturing process. It shouldalso be noted that the chemical composition of thesesteels depends on their thickness. In addition, there arealso significant differences between individual melts ofsteel. Moreover, the manufactures continue the researchof how to modify the chemical composition in terms ofthe optimum combination of mechanical properties andweldability.

Such sophisticated steels also require sophisticatedwelding procedures and welding technologies.19,23 Un-fortunately, the manufactures provide only limited infor-mation about the welding, relating only to conventionalarc-welding processes such gas metal arc (GMA), gastungsten arc (GTA) or submerged-arc welding (SMAW).

At the same time, general guidelines for weldingthermomechanically rolled fine-grained microalloyedsteel grade such as Domex and low-alloy quenched andtempered steel such as Weldox are different. Generally,in the case of thermomechanically rolled fine-grainedmicroalloyed steel, it is recommended not to exceed acertain heat input because it causes a drop in mechanicalproperties and impact toughness. On the other hand, in

the case of low-alloy quenched and tempered steel, theheat input must be within a specific range. Too high aheat input causes a drop in the toughness, like in the caseof the fine-grained microalloyed steel. On the otherhand, too low a heat input can lead to cold cracking of ajoint. Therefore, according to the manufacturer’s instruc-tions, the recommended cooling time t8/5 between tempe-rature ranges from 800 °C to 500 °C is 5 s to 15 s forWeldox 900 to 1300. As laser welding is of an increasingimportance in the industry and the conditions of laserwelding are radically different from arc welding, aresearch of the laser welding of new steel grade Domex960 was undertaken in this work.24,25

2 EXPERIMENTAL PART

The steel chosen for the investigation was the newgeneration of advanced high-strength steel (AHSS)Domex 960, recently introduced to the industry by aSwedish company. The new grade of steel Domex 960 isincluded, by the manufacturer, in the group of thermo-mechanically rolled fine-grained microalloyed steelscovered by the EN 10149-2 standard. However, the me-chanical properties of steel Domex 960 go far beyond thesteels specified in this standard. Additionally, details ofthe manufacturing process for the new steel grade areundisclosed. The investigated steel with the nominal che-mical composition of 0.18 %C, 0.5 %Si, 2.1 %Mn and0.018 %Al in % of mass fractions and balanced Fe hasthe minimum yield strength of 960 MPa and the mini-mum tensile strength of 980 MPa.

Specimens for the test involving laser welding werecut from a 5.0 mm steel plate into coupons with dimen-sions of 100.0 × 100.0 mm, using a 2D laser cuttingmachine with a CO2 generator. Surfaces to be weldedwere sandblasted and then cleaned with acetone. Thetrials of welding were performed by means of a solid-state Yb:YAG disk laser emitted in the continuous-wavemode at a wavelength of 1.03 μm with the maximumoutput power of 3.3 kW. The laser beam was focused to adiameter of 200 μm.

First, the bead-on-plate welds were produced at themaximum output laser power of 3.3 kW and weldingspeeds of (0.5, 1.0, 1.5 and 2.0) m/min. The bead-on-plate welds were produced to simulate the process ofbutt-joint welding and to investigate the influence ofwelding parameters on the penetration depth and weldshape. Based on the bead-on-plate welding trials, theoptimum parameters for butt joints were chosen. Buttjoints were single-side autogenously laser welded at themaximum laser power of 3.3 kW and welding speeds of1.0 m/min and 1.5 m/min, respectively. The specimens tobe welded were mounted to a clamping device to beprotect against distortions. The weld pool was protectedby an argon flow via four cylindrical nozzles of 8.0 mmin diameter and set at an angle of 45° to the joint surface.The flow of argon was kept at 15 L/min. The laser beam

A. KURC-LISIECKA, A. LISIECKI: LASER WELDING OF THE NEW GRADE OF ADVANCED HIGH-STRENGTH STEEL ...

200 Materiali in tehnologije / Materials and technology 51 (2017) 2, 199–204


was focused on the top surface of the specimens to bewelded. When the laser-welding tests were completed,the first the visual inspections (VT) were performedaccording to the procedure of quality control in welding.Next, the metallographic and also mechanical examina-tions were done. The examination of the structure wascarried out by means of optical microscopes (OM) and ascanning electron microscope (SEM). The chemicalcomposition of the base metal was determined with aglow discharge spectrometer (GDS). On the other hand,mechanical tests included a technological bending test, astatic tensile test and a Charpy V-notch test.

3 RESULTS AND DISCUSSION

The trials of bead-on-plate welding with a disk lasershowed that the heat input required for a full penetrationof a 5.0 mm plate of the Domex 960 steel is at least 100

J/mm, at a laser output power of 3.3 kW and a weldingspeed of 2.0 m/min. The width of a weld face producedat the minimum welding heat input is 2.15 mm, while theroot width is 0.8–1.27 mm. In this case the depth/widthratio is over 2.3, indicating that the laser welding modewas the keyhole mode. Additionally, the shape of thefusion zone (FZ) in a columnar or hour-glass configura-tion (X shape) is characteristic for keyhole laser weldingat a high power density of the laser beam. An increase inthe heat input of laser welding, by lowering the weldingspeed at a constant laser power, increases the width of asingle bead-on-plate weld and the width of the heataffected zone (HAZ).

The visual inspections (VT) of the test butt jointsrevealed a proper shape of the welds, and proper rein-forcement of weld faces and roots. Macrographs of thetest joints showed a proper shape of the fusion lines andno internal imperfections (Figure 1a). A high quality ofthe test joints was also confirmed with microscopicobservations (Figure 1b) and mechanical tests. The tech-nological bending test revealed no tendency to crackingof the welded joints, both in the weld and in the HAZ,even at the maximum angle of bending. On the otherhand, the static tensile tests showed high strength oflaser-welded joints at the level of the base metal (BM).




Figure 2: SEM micrographs of the fracture surface of the test jointproduced at: a) the heat input of 198 J/mm and b) the heat input of 132J/mmSlika 2: SEM-posnetek povr{ine preloma preizkusnega spoja izdela-nega z vnosom toplote: a) 198 J/mm in b) vnosom toplote 132 J/mm

Figure 1: a) Macrostructure of the butt joint produced at the weldingspeed of 1.5 m/min, thus the heat input of 132 J/mm, b) and themicrostructure of the weld metal (FZ)BM – base metal, FZ – fusion zone, HAZ – heat-affected zone, HAZCG –coarse-grained region, HAZFG – fine-grained region, HAZPT – partiallytransformed region, B – bainite, M – martensite, �pf – polygonal ferrite, �af –allotriomorphic ferrite

Slika 1: a) Makrostruktura sole`nega zvara, izdelanega s hitrostjovarjenja 1,5 m/min, vnos toplote 132 J/mm in b) mikrostrukturazvarjene kovine (FZ)BM – osnovna kovina, FZ – podro~je taljenja, HAZ – toplotno vplivana cona,HAZCG – podro~je velikih zrn, HAZFG – podro~je drobnih zrn, HAZPT – delnotransformirano podro~je, B – bainit, M – martenzit, �pf – poligonalni ferit, �af –alotriomorfni ferit

All of the tested samples broke out of the joint area inthe base metal at a tensile strength of 1093MPa – 1017MPa. The Charpy V-notch test, performed at room tem-perature, showed that the impact toughness of the testbutt joints is clearly lower compared to the base metal.The results of the impact toughness determined for thebase metal were very consistent, ranging from 128 J/cm2

to 134 J/cm2. The average value was 131.3 J/cm2. Theimpact toughness of the test joint welded at a heat inputof 198 J/mm was in a range of 82 J/cm2 – 98 J/cm2, at theaverage value of 90 J/cm2. So, the impact toughness ofthis joint is as low as 68 % of the base-metal toughness.On the contrary, the impact toughness of the second testjoint welded at a low heat input of 132 J/mm is 70.7J/cm2, so just 53 % of the base-metal toughness.

SEM micrographs of the fracture surfaces of the testjoints are presented in Figure 2. As can be seen, the frac-ture surfaces indicate a typical ductile dimple fracturemode in both cases. So, although the impact toughnessof the test joints is clearly lower compared to the basemetal, the fracture mode is ductile. The results indicatethat the impact toughness of the test butt joints dependsdirectly on the heat input of autogenous laser weldingwithin the investigated range of parameters, i.e., thermalconditions, cooling rates and the structures of the weldmetal and the HAZ.

The cooling times t8/5 between 800 °C and 500 °Cwere calculated for different heat inputs of laser weldingby means of an equation adopted for the conditions ofkeyhole laser welding, as described in details in 4. Forthe heat input of 198 J/mm for the butt laser welding ofthe 5.0 mm steel plates, the calculated cooling time t8/5

was 1.299 s. However, the cooling time in the case of alower heat input of 132 J/mm was just 0.577 s. For com-parison, in both cases, cooling times t8/5 are significantlylower than recommended for the quenched and temperedWeldox grade steel, being in the range of 5s –15 s. How-ever, despite such short cooling times there was notendency to cold (hydrogen) cracking of the test joints(Figure 1).

The microhardness profiles determined on thecross-sections of the welded butt joints show a clearincrease in the microhardness values in the region of theweld metal, i.e., the fusion zone, compared to the micro-hardness of the base metal, which is about 370 HV0.2(Figure 3). The microhardness distribution across the FZis stable and maintained in a range of 360–440, while themean value of microhardness in the FZ region is about404 HV0.2. On the other hand, the distribution of micro-hardness in the HAZ is more complex, so the HAZ canbe divided into at least two areas: the inner HAZadjacent to the FZ and the outer HAZ adjacent to theBM. From the side of the base metal, roughly in themiddle of the HAZ, the microhardness decreasesgradually from 370 to the minimum value of 290–300HV0.2 and then it rises sharply up to over 400 HV0.2 inthe inner HAZ.

The microhardness in individual regions dependsdirectly on the microstructure and this, in turn, dependson the chemical composition and the tendency to hard-ening. The GDS analysis showed that the investigatedsteel contains 0.161 % C, 0.31 % Si, 1.32 % Mn, 0.15 %Cr, 0.05 % Ni, 0.41 % Mo, 0.043 % Al, 0.01 % of V, Ti,Cu and 0.001 % Nb. Based on the determined compo-




Figure 4: SEM micrograph showing the structure of the base metal ofDomex 960 steel at a magnification of 5000×Slika 4: SEM-posnetek strukture osnovne kovine jekla Domex 960 pripove~avi 5000×

Figure 3: Microhardness distribution on the cross-section of the buttjoint produced at the heat input of 132 J/mmSlika 3: Razporeditev mikrotrdote po preseku sole`nega spoja, izdela-nega pri vnosu toplote 132 J/mm

Table 2: Chemical composition of S690QL steel according to EN 10025-6Tabela 2: Kemijska sestava jekla S690QL, skladno z EN 10025-6

C Si Mn B Cr Cu Mo N Nb Ni Ti V Zr0.12 0.8 1.7 0.005 1.5 0.5 0.7 0.015 0.06 2.0 0.05 0.12 0.15

CEV max = 0.65, CET standard = 0.29

sition, the martensitic transformation temperature wascalculated according to the following equation: Ms (°C)= 561 – 474 (% C) – 33 (% Mn) – 17 (% Ni) – 17(% Cr)– 21 (% Mo). The calculated Ms temperature is 429.1 °C.Additionally, the carbon equivalent (CET) was deter-mined according to the following equation: CET = C +(Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40. The value of thecalculated CET is 0.343. Both the high Ms temperatureand the relatively low carbon equivalent indicate that thehardenability of the investigated steel is not very high.

The structure of the base metal of the Domex 960steel is shown in Figure 4. In general, the investigatedsteel has a fine-grained bainitic-martensitic structure. Acloser view at the structure exhibits quite a complexarrangement of different phases. The dominant structuralconstituent is bainite with a significant amount ofneedle-shaped low-carbon martensite. Beside the mainstructural constituents, ferrite and also traces of retainedaustenite can be identified. Additionally, small carbideswith a high dispersion can be identified in the structure.

The structure of the HAZ depends on the distancefrom the fusion line and thus the thermal cycle and therelated cooling rate. The thermal conditions during theliquid-metal solidification, especially in the HAZ, maybe estimated on the basis of calculated cooling times t8/5.The calculated cooling times in the range of 0.6–1.3 arevery short and indicate that the solidification as well asthe cooling rates were very high under the laser-weldingconditions. The HAZ may be divided into three dis-tinguishable regions with different structures (Figure 1a).The first characteristic region of the HAZ is adjacent tothe fusion line. In this region, due to high temperatures, aslight grain growth occurs. In this region, the structure isbainitic with a higher share of ferrite compared to theBM (Figures 1a, 5b). Next, a recrystallized fine-grainedregion with fine-grained bainite as the dominantstructural constituent may be distinguished, cha-racterized by a microhardness in the range of 380–420HV0.2, Figure 3. Finally, a partially transformed andtempered region with a bainitic-ferritic structure may bedistinguished (Figure 5c). A relatively high share offerrite in this region is responsible for a gradualmicrohardness decrease to 280–310 HV0.2, as shown inFigure 3.

The structure of the weld metal of the butt joint pro-duced at a heat input of 132 J/mm is presented in Fig-ures 1b and 5a. The structure consists mainly of bainite,a smaller amount of fine martensitic islands and also offerrite, mainly polygonal (�pf) and allotriomorphic ferrite(�af), as shown in Figures 1b and 5a.

4 CONCLUSIONS

The investigated new steel grade Domex 960 ischaracterized by a low content of alloying elements(microalloyed steel), thus the low carbon equivalent(CET) is 0.343 and the relatively high temperature ofmartensitic transformation (Ms) is about 429 °C. Despitethe low carbon equivalent, the base metal has a fine-grained bainitic-martensitic structure with a microhard-ness of 370–380 HV0.2. The calculated cooling times t8/5

under the investigated laser-welding conditions are veryshort, being in a range of 0.6s –1.3 s, so significantlyshorter than the recommended values for the quenchedand tempered steel grades such as Weldox. Despite avery rapid solidification of the weld metal and cooling,the test joints exhibit a high tensile strength, being at the




Figure 5: SEM micrographs showing the structure of the weld metalof the butt joint produced at the heat input of: a) 132 J/mm, b) theHAZ region adjacent to the fusion zone and c) the recrystallized fine-grained region of the HAZSlika 5: SEM-posnetek structure zvara sole`nega spoja izdelanega zvnosom toplote: a) 132 J/mm, b) podro~je HAZ v bli`ini podro~ja pre-taljevanja in c) rekristalizirano drobnozrnato podro~je HAZ

level of the BM. The impact toughness of the test jointsclearly depends on the welding conditions (heat inputs).Although the impact toughness is lower compared to theBM, the fracture mode is ductile.

Acknowledgements

The article was financed by the University ofD¹browa Górnicza in Poland. Special thanks areextended to Rafa³ Lis, a welding engineer from theWIELTON Company for providing the new steel gradeand also to dr. Wojciech Pakie³a for the experimentalsupport.

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